Photodiodes are semiconductor devices that convert light into either current or voltage. They operate using the photoelectric effect where photons are absorbed, creating electron-hole pairs. Photodiodes can be used in either photovoltaic or photoconductive mode. In photovoltaic mode, a voltage is produced when illuminated, operating similar to solar cells. In photoconductive mode, applying a reverse bias increases the depletion region, improving speed but adding noise. Photodiodes are used in applications such as optical communications, medical devices, and light sensors.